The present invention relates to a touch panel input device, and more particularly, to a touch panel input device which generates coordinate data of a position on a touch panel which a pointing means, such as a finger of a user contacts.
FIG. 1 is for explaining the principle of a touch panel input device. As shown in the drawing, the touch panel input device comprises of a touch panel 11 having a lower layer 111 comprising a resistive conductor and an upper layer 112 comprising a dielectric, a driving circuit 12 for applying a driving signal to predetermined points UR, UL, LL and LR on the lower layer 111, a voltage detecting circuit 13 for converting currents i.sub.UR, i.sub.UL, i.sub.LL and i.sub.LR flowing toward each point into voltage signals V.sub.UR, V.sub.UL, V.sub.LL and V.sub.LR, a switching circuit 14 for switching the voltage signals to generate a coordinate signal, and an analog-to-digital converter (ADC) 15 for converting the coordinate signal into digital coordinate data. Here, R.sub.UR, R.sub.UL, R.sub.LL and R.sub.LR are voltage detecting resistors each having the same resistance value.
In FIG. 1, when there is no physical contact with the surface of the upper layer 112 of the touch panel 11, there is no flow of the currents i.sub.UR, i.sub.UL, i.sub.LL and i.sub.LR since the touch panel 11 is in an electrically floating state. Accordingly, there is no voltage drop across any of the voltage detecting resistors R.sub.UR, R.sub.UL, R.sub.LL and R.sub.LR and all the voltage signals V.sub.UR, V.sub.UL, V.sub.LL and V.sub.LR become "0V." When the user's finger touches the surface of the upper layer 112 of the touch panel 11, the currents i.sub.UR, i.sub.UL, i.sub.LL and i.sub.LR flow towards the finger via the lower and upper layers 111 and 112 of the touch panel 11. Here, the respective currents i.sub.UR, i.sub.UL, i.sub.LL and i.sub.LR are in inverse proportion to the distance between the finger-touch point and each of the points UR, UL, LL and LR. Accordingly, a voltage drop occurs across each of the voltage detecting resistors R.sub.UR, R.sub.UL, R.sub.LL and R.sub.LR and each voltage varies in accordance with the position of the finger. The voltage detecting circuit 13 detects the voltage change and generates each of the voltage signals V.sub.UR, V.sub.UL, V.sub.LL and V.sub.LR. The switching circuit 14 generates X-coordinate and Y-coordinate signals by switching. The analog-to-digital converter 15 converts the coordinate signals into digital coordinate data.
FIG. 2 is a view for explaining the operation of a conventional input device of a touch panel type. As shown in FIG. 2, the conventional touch panel 21 comprises of a lower layer 211 comprising a resistive conductor and an upper layer 212 comprising a dielectric. When a user's finger touches the surface of the upper layer 212, a current due to a driving signal flows via the lower layer 211 and the upper layer 212 toward the finger since a capacitance C.sub.g of the upper layer 212 is present. However, since an air capacitance C.sub.a is present between the upper layer 212 and the finger, noise current flows through the air capacitance C.sub.a. Accordingly, accuracy and precision in relation to the output data of such an input device are lowered. Further, since the surface that the finger touches, i.e., the upper surface of the upper layer 212, is slippery, a user tends to apply force in order to press more firmly.